Process for producing oxide superconductor, oxide superconductor and substrate supporting precursor thereof

ABSTRACT

A method for producing an oxide superconductor by partially melting and solidifying the precursor of the oxide superconductor is a method wherein the precursor is placed on a substrate material containing pure metal or a compound which is meltable in the precursor when the precursor is in a partially molten state, and partially melting and solidifying the precursor in said state.

TECHNICAL FIELD

The present invention relates to a method for producing an oxidesuperconductor, an oxide superconductor and a substrate material forsupporting the precursor thereof. Furthermore, the present applicationis claimed on Japanese Patent Application No. 2003-15208, the content ofwhich is incorporated herein by reference.

BACKGROUND ART

When a large, bulk-shaped oxide superconductor having a diameter ofseveral tens of millimeters or more is solidified by slow-cooling from apartially molten state to promote crystal growth, for example, methodslike that described below are generally employed. One example thereof isthat a bulk-shaped precursor of an oxide superconductor is formed, andthen it is supported by a rod-shaped material, a substrate or asheet-shaped material which is placed below the precursor and iscomposed of a heat-resistant material such as Al₂O₃, YSZ (yttriumstabilized zirconia) or MgO. Alternatively, a method is employed inwhich a mixed powder of a superconducting powder having the compositionof YBa₂Cu₃O_(7-x) (so-called Y123 powder) and an oxide powder having thecomposition of Y₂BaCuO₅ (so-called Y211 powder), or a mixed powder of asuperconducting powder having the composition of YbBa₂Cu₃O_(7-x)(so-called Yb123 powder) and an oxide powder having the composition ofYb₂BaCuO₅ (so-called Yb211 powder), which has a peritectic temperatureequal to a peritectic temperature of a precursor of a superconductor orlower, is placed on a metal mount or the like.

When a precursor of an oxide superconductor is placed directly on asupport member such as a dish or crucible composed of a heat-resistantmaterial such as platinum or Al₂O₃ (alumina) and heated to a temperatureat which the precursor is partially molten, the precursor in thepartially molten state reacts with materials comprised in theheat-resistant material and adheres to the dish or crucible. When theprecursor is solidified, the precursor is subjected to large stress dueto the difference in thermal expansion coefficient between the precursorand the dish or crucible, resulting in the formation of cracks which areundesirable for an oxide superconductor. The aforementioned generalmethods are used to avoid these problems.

Namely, in the aforementioned methods, the precursor in partially moltenand solidified state is supported with a support member wherein thecomposition thereof is as close as possible to the composition of theoxide superconductor. As a result, stress loading caused by thedifference in thermal expansion coefficient when the precursor issolidified, is reduced, and crack formation is prevented as much aspossible in the oxide superconductor obtained with these methods.

In addition, as described in Japanese Unexamined Patent Application,First Publication No. Hei 5-229820, a technology is provided in which aprecursor in a partially molten state is supported by floating on moltensilver in a metal dish, and then the precursor is solidified from thispartially molten state. Here, the silver hardly reacts with an oxidesuperconductor. It is described in the document that, due to the abovecharacteristic, the resulting oxide superconductor can be easily removedfrom a solid of the silver after a melting and solidifying theprecursor.

However, in the previously described methods using the rod-shapedmaterial, a substrate or a mixed powder which consists of a materialhaving a composition which is similar to the composition of the oxidesuperconductor, there was still the problem of susceptibility toformation of cracks by reason explained below, although the risk ofcrack formation is lower than the method using a support member such asa dish or crucible composed of platinum or Al₂O₃ (alumina).

Here, when considering the partially molten state of an oxidesuperconductor, a superconductor powder having the composition ofYBa₂Cu₃O_(7-x) decomposes at the peritectic temperature or higher as inthe manner of the following formula (I).2YBa₂Cu₃O_(7-x) (Y123)=Y₂BaCuO₅ (Y211)+L(3BaCuO₂+2CuO)  (I)

In this formula (I), L represents a liquid phase. X represents theamount of oxygen deficiency in the lattice thereof.

The superconductor in the partially molten state has a liquid phase.Accordingly, when the supporting methods of the prior art are used,problems are thought to occur such that deformation of a lower portionof a precursor in the partially molten state is caused due to its weightwhen the precursor supported by several rod-like support members issoftened, or the bottom of the precursor adheres to the support memberas a result of a reaction between the precursor and the support member,or the like. In addition, in the methods in which an oxidesuperconductor is supported by a support member or mixed powder whichhas a composition similar to that of the oxide superconductor, asolidified portion (portions in which a reaction has proceededspontaneously) forms easily that consists of a composite oxide having acomposition that contains rare earth elements, and a subtle differencein the coefficients of thermal expansion between the solidified portionand an oxide superconductor which is surrounding the solidified portionis easily formed. In actuality, the inventors of the present inventionobtained experimental results demonstrating that cracks form easily inthe vicinity of the interface between the aforementioned solidifiedportion composed from composite oxide (portions in which a reaction hasproceeded spontaneously) and the oxide superconductor portion which issurrounding the solidified portion, when this type of oxidesuperconductor was attempted to be produced by a partial melting andsolidification method wherein solidification is conducted from apartially molten state. Here, in the present invention, “partiallymolten” state means a state in which the 123 phase is melted, while the211 phase remains in the solid phase and is dispersed in the molten 123phase.

Among the various types of oxide superconductors, RE-Ba—Cu—O based oxidesuperconductors (where RE contains a rare earth element) have a highcritical temperature and are widely known.

In this type of oxide superconductors, an oxide superconductor wherein Yis used as a rare earth element thereof and the composition thereof isYBa₂Cu₃O_(7-x) is considered that it has a low risk of crack propagationthroughout entire bulk of the oxide superconductor, even if fine cracksare partially formed, when the bulk of the oxide superconductor isproduced by a partial melting and solidification method. In actuality, abulk of a Y—Ba—Cu—O based superconductor has been produced having a highcritical current density and a diameter of about 100 mm.

However, for example, in the case of an oxide superconductor in which Ndis used as the rare earth element, when fine cracks are formed, thecracks easily propagate throughout the oxide superconductor.Accordingly, if partial cracks form in the oxide superconductor, thecracks propagate throughout the oxide superconductor and break the oxidesuperconductor, or the oxide superconductor tends to have remarkably lowsuperconductivity characteristics and large cracks penetrating throughthe entire superconductor. For example, when a Nd based bulk havingsuperior superconductivity characteristics is produced using the partialmelting and solidification method, a current production limit is aproduction of a bulk having a diameter of about 30 mm. However, such abulk having the diameter is unable to be produced at satisfactory yielddue to the presence of cracks.

It is understood from documents describe below regarding the difficultyin obtaining large, bulk-shaped oxide superconductors that are free ofcracks and other defects, that “As the size of bulk materials made toundergo crystal growth becomes larger, the length of time required forcrystal growth increases. As a result, prolonged heat treatment isconducted in the partial molten state, and compositional variations andthe like are caused due to a loss in liquid phase components, orcontamination caused from materials contained in the substrate or thelike, thereby making it difficult to obtain high-quality crystals.”Examples of these documents include a document published by D. A.Cardwell entitled “Processing, microstructure and characterization ofartificial joint in top seeded melt grown Y—Ba—Cu—O” in “Institute ofPhysics Publishing Superconductor Science and Technology, 15 (2002)639-647”, a document published by Lihua Chen entitled “Joining ofmelt-textured YBCO: A direct contact method” in “Institute of PhysicsPublishing Superconductor Science and Technology, 15 (2002) 672-674”,and a document described by Naomichi Sakai, et al. entitled“Microgravity Superconductor Production Project” in “Cryogenics, 34, 11(1999) p. 563”.

In consideration of the aforementioned problems, an object of thepresent invention is to provide a technology which enables a productionof a large, bulk-shaped oxide superconductor that is free of defects,wherein cracks which are caused by a difference in the coefficients ofthermal expansion between an oxide superconductor and a support memberare not formed when a production of an oxide superconductor is carriedout using a partial melting and solidification methods.

Another object of the present invention is to provide a preferablesubstrate material for supporting a precursor of an oxidesuperconductor, which is usable for producing a large, bulk-shaped oxidesuperconductor free from cracks by using a partial melting andsolidification method.

In addition, another object of the present invention is to provide alarge, bulk-shaped oxide superconductor free from cracks by using apartial melting and solidification method.

Moreover, another object of the present invention is to provide atechnology that enables a production of a bulk-shaped Nd based oxidesuperconductor having a diameter of about 30 mm or more, which iscurrently the largest size in the world.

DISCLOSURE OF THE INVENTION

A first aspect of the present invention is a method for producing anoxide superconductor comprising: placing a precursor of an oxidesuperconductor on a substrate material containing pure metal or acompound which is meltable in the precursor when the precursor is in apartially molten state, and producing the oxide superconductor bypartial melting and solidifying the precursor in said state.

A second aspect of the present invention is a substrate material forsupporting a precursor of an oxide superconductor, wherein the substratematerial is used in a method for producing an oxide superconductor bysolidification from a partially molten state of a precursor of anRE-Ba—Cu—O based oxide superconductor wherein RE represents a rare earthelement, and the substrate material consists of a material whichcontains Ba or Cu but does not contain a rare earth element in thepartially molten state.

A third aspect of the present invention is an RE-Ba—Cu—O based oxidesuperconductor, wherein RE represents a rare earth element, and theoxide superconductor includes a portion which is solidified aftermelting and contains one of or both of Ba and Cu but does not contain arare earth element on an outside face of the oxide superconductor, andthe solidified portion corresponds to a substrate material which is usedfor supporting a precursor of the RE-Ba—Cu—O based oxide superconductorwhen the precursor was partially melted and solidified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration which explains a state in which themethod of the present invention is carried out using a precursor of anoxide superconductor.

FIG. 2 is a schematic illustration which shows a bulk-shaped oxidesuperconductor obtained according to the method of the presentinvention.

FIG. 3 is a schematic illustration of a bottom of an oxidesuperconductor which has a composition of NdBa₂Cu₃O_(7-x) and isobtained in Examples as a comparative material.

FIG. 4 is a schematic illustration of a bottom of an oxidesuperconductor, which has a composition of NdBa₂Cu₃O_(7-x) and isobtained in Examples according to the present invention.

FIG. 5A is a graph showing a trapped magnetic field distribution of atop surface of an oxide superconductor obtained in Examples according tothe present invention.

FIG. 5B is a graph showing the trapped magnetic field distribution of abottom surface of the oxide superconductor obtained in Examplesaccording to the present invention.

FIG. 6A is a graph showing the trapped magnetic field distribution of atop surface of an oxide superconductor obtained in Examples as acomparative material.

FIG. 6B is a graph showing the trapped magnetic field distribution of abottom surface of the oxide superconductor obtained in Examples as acomparative material.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention relates to a method for producing an oxidesuperconductor having a high critical current density by using a partialmelting and solidification method, and an oxide superconductor which isproduced according to that method. More particularly, the presentinvention relates to a technology wherein a large oxide superconductorin which cracks and other defects do not form in the resulting oxidesuperconductor can be produced.

In order to achieve the aforementioned objects, the present inventionprovides a method for producing an oxide superconductor wherein aprecursor of an oxide superconductor is solidified after partialmelting. In the method, the precursor is placed on a substrate materialcontaining a compound or pure metal which is able to melt in theprecursor in a partially molten state, and then an oxide superconductoris produced by partial melting and solidifying the precursor from thisstate.

The partial melting and solidification is carried out while a precursorwhich is partially melted is supported with a substrate materialcomprising a compound or pure metal that can be melted in the partiallymelted precursor. Consequently, even if a portion of materials comprisedin the substrate material melts in the precursor which is heated to ahigh temperature and is in the partially molten state, a portion atwhich a reaction proceeds preferentially does not occur in theprecursor. Accordingly, an oxide superconductor which is obtained afterpartially melting and solidification hardly has cracks which are formeddue to a difference in coefficients of thermal expansion.

Here, a partial melting and solidification method is known as aproduction method described below. After obtaining a raw material mixedmolding (precursor) which is molded after mixing a plurality ofcompounds of elements which form an oxide superconductor, this precursoris melted by heating at a temperature equal to or higher than themelting point thereof. Subsequently, the precursor is cooled graduallywhile applying a temperature gradient, and a seed crystal is placed in aportion of the precursor at a temperature immediately before atemperature at which crystallization of the precursor is caused, andcrystal is grown inside the precursor using the seed crystal as startingpoint. In this manner, an oxide superconductor is obtained having a wellaligned crystal structure and superior superconductivitycharacteristics.

In order to achieve the aforementioned objects, the present inventionhas a characteristic in which a substrate material is used and thesubstrate material consists of a pure metal or compound which ismeltable in a partially molten precursor uniformly. The pure metal andcompound does not allow the formation of a region, in which reactionproceeds spontaneously to form stress concentration cracks in the oxidesuperconductor. The cracks are resulting from a difference in thecoefficients of thermal expansion between the substrate material and theoxide superconductor.

The precursor in a partially molten state is supported with thesubstrate material which can be uniformly melted in a partially moltenprecursor and does not cause the formation of a region in which areaction proceeds spontaneously. Accordingly, even if a region ofcomposite materials included in the substrate material melts in theprecursor in a partially molten state which is heated to a hightemperature, a uniform molten state can be achieved, and a region atwhich a reaction proceeds spontaneously is not formed in the precursor.Therefore, it is difficult for cracks to form and exist in the precursordue to a difference in the coefficients of thermal expansion, afterconducting partial melting and solidification process of the precursor.

In order to achieve the aforementioned objects, the present inventionhas a characteristic in which the oxide superconductor is a RE-Ba—Cu—Obased oxide superconductor, wherein the RE represents a rare earthelement, and the substrate material contains Ba or Cu but does notcontain a rare earth element in a molten state.

Specific examples of the oxide superconductor can be selected fromRE-Ba—Cu—O based oxide superconductors, and materials which form thesubstrate material can be selected from materials that contain Ba or Cu,but does not contain a rare earth element.

A region at which a reaction proceeds preferentially comprises acomposite oxide which has the different composite ratio than the targetoxide superconductor and comprises a rare earth element, Ba, Cu and Owhich form an oxide superconductor. The region has a differentcoefficient of thermal expansion than the body of an oxidesuperconductor obtained by a partially melting and solidificationmethod. Accordingly, the difference of the coefficients of thermalexpansion causes the formation of cracks during solidification followingpartially melting. Hence, an oxide superconductor in which cracks do notform can be provided by combining a precursor and a substrate materialso that a region at which a reaction proceeds preferentially does notgenerate therein even after partial melting and solidification.

In order to achieve the aforementioned objects, the present inventionhas a characteristic in which, after placing an intermediate layercontaining at least one selected from the group consisting of Y₂O₃,Yb₂O₃, Er₂O₃, Ho₂O₃, Dy₂O₃, Eu₂O₃, Sm₂O₃, Gd₂O₃, ZrO₂, Al₂O₃, BaZrO₃,MgO and yttrium stabilized zirconia (YSZ) on a mount made of aheat-resistant material, a substrate material is placed on the mount,and a precursor of the oxide superconductor is placed on the substratematerial when partial melting and solidification of the precursor isconducted.

The intermediate layer hardly reacts at all with the mount duringpartial melting and solidification, or a reaction between theintermediate layer and the mount can be suppressed. Furthermore, thesubstrate material provided on the intermediate layer does not reactwith the precursor. Accordingly, the precursor does not react with themount during partial melting and solidification process, and unnecessaryportions at which a reaction proceeds spontaneously are not formed inthe precursor. As a result, cracks do not form in the resulting oxidesuperconductor which is obtained after partial melting andsolidification process of the precursor.

In order to achieve the aforementioned objects, the present invention isto use a substrate material, and specific examples of the substratematerial include the substrate materials comprising one of or two ormore kinds of a pure metal of Ba (melting point: 725° C.) or Cu (meltingpoint: 1083° C.), alloys, or oxides, composite oxides, carbonates,sulfides, sulfates, chlorides, hydroxides and nitrates of Ba or Cu.

In order to achieve the aforementioned objects of the present invention,one of or two or more kinds of BaO (melting point: 1920° C.), CuO(melting point: 1026° C.), Cu₂O (melting point: 1232° C.), BaCuO₂(melting point: 980° C.), BaCO₃ (melting point: 811° C.), CUCO₃ (meltingpoint: 220° C.), BaS (melting point: 1200° C.), CuS (melting point: 220°C.), BaSO₄ (melting point: 1580° C.), CuSO₄ (melting point: 200° C.),BaCl₂ (melting point: 963° C.), CuCl (meltingpoint: 430° C.), CuCl₂(meltingpoint: 620° C.) Ba(OH)₂ (melting point: 78° C.), Cu(OH)₂(melting point: 220° C.), Ba(NO₃)₂ (melting point: 592° C.) and Cu(NO₃)₂(melting point: 114.5° C.) can be selected and used in the presentinvention, as the aforementioned oxides, composite oxides, carbonates,sulfides, sulfates, chlorides, hydroxides or nitrates of Ba or Cu.

In order to achieve the aforementioned objects, specific examples of asubstrate material used in the present invention include substratematerials comprising one of or two or more kinds of noble metals such asAg (melting point: 962° C.), Au (melting point: 1065° C.), Pt (meltingpoint: 1772° C.) and Pd (melting point: 1554° C.), or an oxide of thenoble metals such as Ag₂O (melting point: 300° C.) and PtO₂ (meltingpoint: 450° C.) These pure metals or compounds either certainly melt ina precursor which is in the partially molten state or remain in theiroriginal form without melting, when a partially melting andsolidification process is conducted in a heat treatment in the range of1000 to 1200° C.

A substrate material for supporting a precursor of an oxidesuperconductor of the present invention is the material which can bemelted in a partially molten state. The substrate material can be usedin a process for producing an oxide superconductor wherein the processis conducted by solidification of a precursor of an RE-Ba—Cu—O basedoxide superconductor from a partially molten state (RE represents a rareearth element). The substrate material consists of a material whichcontains Ba or Cu but does not contain a rare earth element in themolten state.

One of or two or more kinds of a pure metal of Ba or Cu, oxides,composite oxides, carbonates, sulfides, sulfates, chlorides, hydroxidesor nitrates of Ba or Cu, can be used as constitutive materials includedin the aforementioned substrate material.

Examples of these compounds which can be contained in the substratematerial comprise the aforementioned various compounds listed above.

In order to achieve the aforementioned objects, the present inventionhas a characteristic in which an RE-Ba—Cu—O based oxide superconductor(RE represents a rare earth element) which is produced by partialmelting and solidifying a precursor of the oxide superconductor isprovided. A molten and solidified region, which contains one of or bothof Ba and Cu but does not contain a rare earth element, is formed andcontained as a portion of the oxide superconductor. The precursor wassupported by the portion, when the precursor was partially melted andsolidified.

When an oxide superconductor is produced by melting and solidifying aprecursor which is supported with a substrate material that can melt inthe precursor but does not contain a rare earth element, a molten andsolidified region which does not contain a rare earth element butcontains either or both of Ba and Cu is formed in a region at which theprecursor was supported. A region in which a reaction has proceededspontaneously is not formed in the molten and solidified portion of theoxide superconductor, which is produced by partial melting andsolidification. Therefore, there are few places having differentcoefficients of thermal expansion in the resulting oxide superconductor,and an oxide superconductor having superior superconductivitycharacteristics is obtained in which the cracks caused by thedifferences in coefficient of thermal expansion during solidification donot form.

In order to achieve the aforementioned objects of the present invention,one of or two or more kinds of a noble metal such as Ag, Au, Pt or Pdmay be additionally contained in the aforementioned molten andsolidified portion. In order to achieve the aforementioned objects, thepresent invention has a characteristic in which the aforementionedmolten and solidified portion is formed in the bottom of the precursorof the aforementioned oxide superconductor.

FIG. 1 is a schematic illustration for explaining a state in which theproduction method of the present invention is carried out, and it showsa state in which a precursor 5 of an oxide superconductor is placed on aplate-shaped mount 1 made of a heat-resistant material via anintermediate layer 2 and a substrate material 3 which are interposedbetween the mount and the precursor.

Although the outer shape of the aforementioned mount 1 is a plate asshown in FIG. 1, the mount 1 may have any arbitrary shape insofar as ithas a shape that allows placement of precursor 5, and it may have theshape of a board, crucible or the like. Since this mount 1 should beable to withstand the temperature at which a melting and solidificationmethod as described later is carried out on precursor 5 (for example,950 to 1200° C.), the mount is composed of heat-resistant material suchas Al₂O₃ (alumina), MgO or YSZ (yttrium-stabilized zirconia), and anexample of which is heat-resistant ceramics.

In this example, the aforementioned intermediate layer 2 and substratematerial 3 are formed into a layered form using an aggregate of powderthereof, thereafter, the intermediate layer 2 is placed on the mount 1,and the substrate material 3 is placed on the intermediate layer 2, andthe precursor 5 is placed on the substrate material 3. The intermediatelayer 2 and substrate material 3 are placed on the mount 1 by spreadingout each powder thereof, which is described later, on the mount in thatorder using a providing tool such as a square form.

The aforementioned intermediate layer 2 consists of Y₂O₃, Yb₂O₃, Er₂O₃,Ho₂O₃, Dy₂₀₃, EU₂O₃, Sm₂O₃, Gd₂O₃, ZrO₂, Al₂O₃, BaZrO₃, MgO oryttrium-stabilized zirconium (YSZ). More specifically, it is formed intoa layer with a powder selected from these materials. The intermediatelayer 2 is provided as a reaction inhibiting layer to prevent a reactionbetween a mount 1 composed of a heat-resistant material such as aluminaand a substrate material 3 made of a material to be described later.Furthermore, although the intermediate layer 2 has a layered structurein which a powder is spread out, another intermediate layers such asthose in which the aforementioned material has been processed into theform of a sheet or plate in advance can also be used.

It is preferable that a material included in the aforementionedintermediate layer 2 is a material which has low reactivity with a mount1 as those listed above, and simultaneously has low reactivity with asubstrate material 3 as well.

A material which does not react with precursor 5 but melts when theprecursor 5 is in the partially molten state can be used for theaforementioned substrate material 3. Examples of the material include apure metal powder of Ba or Cu, and a powder of a compound containing Baand Cu.

More specifically, a material which consists of one of or two or morekinds of an oxide powder, composite oxide powder, carbonate powder,sulfide powder, sulfate powder, chloride powder, hydroxide powder ornitrate powder of Ba or Cu can be used as the substrate material.

Moreover, one of or two or more kinds of powder of BaO (melting point:1920° C.), CuO (melting point: 1026° C.), Cu₂O (melting point: 1232°C.), BaCuO₂ (melting point: 980° C.), BaCO₃ (melting point: 811° C.),CuCO₃ (melting point: 220° C.), BaS (melting point: 1200° C.), CuS(melting point: 220° C.), BaSO₄ (melting point: 1580° C.), CuSO₄(melting point: 200° C.), BaCl₂ (melting point: 963° C.), CuCl (meltingpoint: 430° C.), CuCl₂ (melting point: 620° C.), Ba(OH)₂ (melting point:78° C.), Cu(OH)₂ (melting point: 220° C.), Ba(NO₃)₂ (melting point: 592°C.) or Cu(NO₃)₂ (melting point: 114.5° C.) can be selected and used asspecific examples of the aforementioned oxide powder, composite oxidepowder, carbonate powder, sulfide powder, sulfate powder, chloridepowder, hydroxide powder or nitrate powder of Ba or Cu.

When a compound having a low melting point is selected from them andused for the substrate material, the objects of the present inventioncan be achieved such that the compound is certainly decomposed at theheating temperature during a partial melting and solidification process,components such as C, S, Cl, OH and NH₃ are removed by decomposition, Baor Cu are uniformly melted in the precursor 5 which is in the partiallymolten state, and a molten region, in which there is no formation of aportion at which a reaction proceeds spontaneously, is formed. Inaddition, even in the case of a material having a high melting pointthat does not undergo complete thermal decomposition at a heatingtemperature in melting and solidification process, when it is an oxidebased powder, a compound can be formed due to a mutual reaction bysimultaneously using the oxide based powder and another powder sinceoxygen is contained in the oxide superconductor, and uniform melting isachieved at the heating temperature of melting and solidificationprocess, and thus problems are not caused.

In addition to the aforementioned pure metals and compounds, specificexamples of metals and compounds which are usable for the aforementionedsubstrate material 3 include one of or two or more kinds of noble metalssuch as Ag (melting point: 962° C.), Au (melting point: 1065° C.), Pt(melting point: 1772° C.) and Pd (melting point: 1554° C.), alloys ofthese noble metals, and oxides of the aforementioned noble metals, suchas Ag₂O (melting point: 300° C.) and PtO₂ (melting point: 450° C.).These pure metals or compounds either certainly melt in the partialmolten state of the precursor 5, or remain in their original formwithout melting at all, when a partial melting and solidificationprocess is carried out by heat treating within the range of 1000 to1200° C. In the case of the metals or compounds that remain in theiroriginal form, some elements thereof may have the possibility ofdispersing in the precursor 5 which is in the partially molten state.However, even if the elements are dispersed, they can melt in precursor5, and therefore, a portion described later in which a reaction hasproceeded spontaneously can be made not to occur in the final form ofthe oxide superconductor.

The precursor 5 of the aforementioned oxide superconductor is a compactof a mixture of raw materials wherein the mixture has the same or asimilar composition as the composition of the target oxidesuperconductor. An example thereof is a RE-Ba—Cu—O type composition(wherein RE represents a rare earth element including Y (one of or twoor more kinds of La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb or Lu)). Here,in the case the composition of the target oxide superconductor isNdBa₂Cu₃O₇-x, for example, the precursor 5 is obtained such that apowder having a composition of NdBa₂Cu₃O_(7-x) and a powder having acomposition of Nd₄Ba₂Cu₂O₁₀ are mixed and compressed followed bysintering in pure oxygen. In the case the target oxide superconductorhas a composition of SmBa₂Cu₃O_(7-x), an example of the precursor 5 isobtained such that a powder having a composition of SmBa₂Cu₃O_(7-x) anda powder having a composition of Sm₂BaCuO₅ are mixed and compressed. Inthe case the composition of the target oxide superconductor isGdBa₂Cu₃O_(7-x), an example of precursor 5 is obtained such that apowder having a composition of GdBa₂Cu₃O_(7-x) and a powder having acomposition Gd₂BaCuO₅ are mixed and compressed.

In the case of the state shown in FIG. 1, the precursor 5 isheat-treated according to a partial melting and solidification method.

Here, a partial melting and solidification method comprises, mixing aplurality of compounds having each element that is included in anRE-Ba—Cu—O based oxide superconductor, compacting the mixture to obtaina molding of the mixture of raw materials, and then heating and meltingthis molding at a temperature equal to or higher than its melting pointto drive the molding into a partially molten state while retaining theform of the molding. Next, the molding is cooled slowly while applying atemperature gradient, a seed crystal is placed on top of the precursorat the temperature which is slightly higher than the crystallizationtemperature, and crystal is grown inside the precursor using the seedcrystal as a stating point. These steps constitute a method which isknown as a production method for obtaining an oxide superconductor.

Namely, the precursor 5 is put into a partially molten state by heatingto a temperature slightly higher than the melting point of the precursor5 so that the form of precursor 5 itself is not disturbed. In addition,an oxygen atmosphere in which a minute amount of oxygen is supplied inan inert gas is used for the heating atmosphere. For example, an argongas atmosphere having an oxygen concentration of 1% can be used for aheating atmosphere of the present invention.

The heating temperature at this time can vary slightly depending on thecomposition of the target oxide superconductor, or the components of theatmospheric gas used in the heat treatment. In an inert gas atmospherecontaining 1% oxygen, the range of the heating temperature is generally1000 to 1200° C. when an Nd based oxide superconductor is formed, orgenerally 970 to 1200° C. when other types of oxide superconductors areformed.

Once the precursor 5 has been put into a partially molten state, a seedcrystal is placed on the top of the precursor 5 at the temperature,which is slightly higher than the crystallization temperature. Then, thetemperature is further slowly lowered in a stepwise manner, and theprecursor 5 is held at a predetermined temperature for several tens ofhours and then it is furnace-cooled. For example, a seed crystal isplaced on the top of a precursor 5 after the precursor is graduallycooled to a temperature several tens of degrees lower than thetemperature at which the precursor is in the partially molten state.Subsequently, by slowly cooling the aforementioned precursor to atemperature an additional several tens of degrees lower, and holding theprecursor at that temperature for several tens of hours and thenfurnace-cooled, an oxide superconductor 6 like that shown in FIG. 2 isobtained. For example, if a temperature at which a precursor ispartially molten is 1100° C., precursor 5 is cooled to 1010° C., andthen a seed crystal is placed on the surface of the precursor 5.Further, it is slowly cooled to 1000° C. Then, after slowly cooling to989° C., it is held at that temperature for 60 hours, and then furnacecooled.

In the partial molten state, based on the previously described formula(I), a decomposition into Y₂BaCuO₅ (Y211 phase) and L (liquid phase)(3BaCuO₂+2CuO) is taking place inside the precursor 5. The liquid phasetends to push out the Y211 phase towards the bottom (toward a side whichis away from the seed crystals), when a crystal of the oxidesuperconductor having the composition ratio of YBa₂Cu₃O_(7-x) (Y123phase) grows by using the seed crystal as a starting point. As a result,the whole body of the precursor 5 is crystallized, and a bulk of anoxide superconductor having the composition YBa₂Cu₃O_(7-x) (Y123 phase)is obtained.

In the case of carrying out the partial melting and solidificationmethod in this manner, a portion of the substrate material 3 also meltsat the bottom side of the precursor 5 if the precursor 5 is allowed toremain in the partially molten state for a long period of time.Materials comprised in the substrate material 3 are mainly Ba or Cu andable to melt into precursor 5 in the partially molten state. Therefore,even if the substrate material melts, the substrate material has thesame component as the liquid phase (3BaCuO₂+2CuO) which is formed withinthe precursor 5 when the precursor is in the partially molten state.Accordingly, it does not contaminate the precursor 5 in the partiallymolten state. Namely, there is no formation of a spontaneously nucleatedregion (composite oxide portion) that has the composition ratiodifferent from the target ratio of the target oxide superconductorwherein the difference is caused by a reaction which proceeds separatelyfrom a reaction which forms the oxide superconductor. In addition, amolten and solidified portion 7 that is produced due to a moltenreaction between a substrate material 3 and an oxide superconductor isformed as shown in FIG. 2 on the bottom side of an oxide superconductor6.

When slowly cooling a precursor 5 in the partially molten state andsolidifying it followed by crystal growth, since a spontaneouslynucleated region, which has a coefficient of thermal expansion differentfrom that of the oxide superconductor having the target composite ratio,is not formed in precursor 5, the oxide superconductor is not subjectedto thermal stress caused by the difference in the coefficients ofthermal expansion between a spontaneously nucleated region and the oxidesuperconductor portion. Consequently, an oxide superconductor 6 in whichcracks are not formed in the cooling stage is obtained without defects.

Since the oxide superconductor 6 that is free of cracks can be produced,according to the present invention, an oxide superconductor havingsuperior superconductivity characteristics and free of cracks and otherdefects can be produced. Furthermore, since the resulting oxidesuperconductor is free of cracks, in the case of, for example, coolingan oxide superconductor to the temperature of liquid nitrogen andapplying an external magnetic field followed by removing the externalmagnetic field to trap the magnetic field in the oxide superconductor,the oxide superconductor is able to demonstrate performance of showing ahigh peak of the trapped magnetic field. In addition, since theresulting oxide superconductor does not contain cracks, the trappedmagnetic field has a single peak. However, in the case of an oxidesuperconductor containing multiple cracks, the measurement of thedistribution of the trapped magnetic field thereof reveals that thetrapped magnetic field has divided into multiple peaks, and at the sametime, the trapped magnetic peaks are extremely low. Naturally, acrack-free oxide superconductor is superior to an oxide superconductorthat contains cracks in terms of critical current density as well.

A molten and solidified portion 7 formed in oxide superconductor 6 isshown in FIG. 2 wherein it has been formed over the entire bottomsurface. However, molten and solidified portion 7 is not always formedover the entire bottom surface. The portion may only be formed at aportion of the bottom surface depending on the supported state ofprecursor 5, the temperature, the production conditions and so forth.For example, in the embodiment of FIG. 1, precursor 5 is supported witha layer in which a powder is provided. However, if a rod-shapedsubstrate material, frame-shaped substrate material, or pedestal-shapedsubstrate material or the like is used to support precursor 5 instead ofthe powder layer, a molten and solidified portion partially forms on thebottom side of the oxide superconductor, mainly at the portion wherethere is contact between the substrate material and precursor 5. Inaddition, in the previously described ideal reaction state, a rare earthelement, which is one of the chemical elements of the target oxidesuperconductor, is not contained in the molten and solidified portion 7which is formed at a portion of the interface between precursor 5 andsubstrate material 3. However, a certain amount of rare earth elementsmay be partially contained depending on the reaction conditions.

Next, in the present embodiment, an intermediate layer 2 is interposedbetween a mount 1 made of a heat-resistant material and a substratematerial 3 in order to eliminate or inhibit the reaction between them.Accordingly, since the substrate material 3 does not react with theintermediate layer 2, and the substrate material 3 is composed of amaterial that does not form a spontaneously nucleated region by meltingwith the precursor 5 formed on the substrate material 3, a portion inwhich a reaction proceeds spontaneously can be reliably prevented fromoccurring in the precursor 5. Furthermore, the material used forintermediate layer 2 is required to withstand heat and not to melt whena melting and solidification method is carried out. Even if a portion ofan intermediate layer 2 melts, it is preferable that the intermediatelayer 2 hardly reacts or a reaction with mount 1 can be inhibited, andthe intermediate layer hardly reacts or the reaction of the intermediatelayer with substrate material 3 can be inhibited.

In the case of forming the aforementioned substrate material 3 into theshape of a rod or frame, a plurality of rod-shaped substrate materials 3may be placed directly on a mount 1, and a precursor 5 may be placedthereon followed by carrying out a melting and solidification method. Inthat case, the reaction with the mount 1 can be inhibited by heating therod-shaped substrate material so as not to melt. In addition, if thesubstrate material has an adequately large size, even if a reactionoccurs at a region in contact with the mount, an intermediate layer 2 isnot necessary to be used since the reaction of the mount 1 no longer hasan effect on the portion at which the precursor 5 is supported with thesubstrate material.

Naturally, an intermediate layer 2 may be affixed to the mount 1 side ofthe substrate material formed into the shape of a rod, frame or thelike, or may be arranged at the portion that makes contact with mount 1,in order to inhibit the reaction between the substrate material formedinto the shape of a rod or frame and mount 1.

EXAMPLES Example 1

A pulverized sintered material having the compositional ratio ofNdBa₂Cu₃O_(7-x) (Nd123) and a pulverized oxide having a composition ofNd₄Ba₂Cu₂O₁₀ were weighed and mixed so that their blending ratio was5:1. Moreover, 10% by weight of silver oxide (Ag₂O) powder was added forthe purpose of inhibiting the variation in mechanical strengths of abulk-shaped oxide superconductor to be produced, and a mixture thereofwas obtained. This mixture was mixed and pulverized for 3 hours with apowder mixing device to obtain a mixed powder of raw materials. Thesilver oxide added has the effect of lowering the peritectic temperatureof the aforementioned Nd123. In addition, since the peritectictemperature of a bulk having a composition of NdBa₂Cu₃O_(7-x) is thehighest among RE-Ba—Cu—O based rare earth oxide superconductors, andsince suitable seed crystals are not known in the state of not addingsilver, 10% silver oxide was added by weight in this example.

Uni-axial pressing was carried out on the aforementioned mixed powder ofraw materials at a pressure of 1.5 tons to mold into a pellet (diameter:40 mm, thickness: 13 mm) Subsequently, this pellet was subjected to coldisostatic pressing treatment at a pressure of 2 tons/cm² to produce twobulks. These bulks were then sintered at 1040° C. in pure oxygen toobtain two precursors.

Next, a Y₂O₃ powder layer (intermediate layer) was formed on an alumina(Al₂O₃) boat (mount). This Y₂O₃ powder layer was formed by spreadingY₂O₃ powder to a thickness of about 2 mm on the aforementioned boatusing a square form. Next, BaCuO₂ powder was spread to a thickness ofabout 2 mm on the Y₂O₃ powder layer using a square form to form a BaCuO₂powder layer (substrate material), and the aforementioned precursor wasplaced thereon to obtain a sample of the present invention.

In addition, the same Y₂O₃ powder layer as that described above wasformed on an alumina (Al₂O₃) boat, and a mixed powder, consisting of apowder having the composition of YbBa₂Cu₃O_(7-x) (Yb123) and a powderhaving a composition of Yb₂BaCuO₅ was spread to a thickness of about 2mm using a square form thereon followed by placing the aforementionedprecursor thereon to prepare a comparative material.

The sample of the present invention and comparative material were placedin an argon gas atmosphere containing 1% oxygen, heated to 1100° C. tolet the precursors into a partially molten state, and then held at 1100°C. for 1 hour. Subsequently, the sample and comparative material werecooled to 1010° C. over 2 hours, and Nd based seed crystals to whichsilver oxide had not been added were placed on the top surface of theprecursors in the partially molten state followed by cooling to 1000° C.over 5 minutes. Next, the sample and comparative material were slowlycooled to 989° C. at the rate of 1° C./hour, and held for 60 hours atthis temperature (989° C.), and then furnace-cooled to obtaindisk-shaped oxide superconductors having a diameter of 30 mm and athickness of 10 mm.

The resulting oxide superconductors were taken out of the furnace andobserved. A spontaneously nucleated region (composite oxide portion) atwhich Yb was preferentially reacting with the precursor in partiallymolten state was confirmed at multiple locations on the bottom of theoxide superconductors of the comparative materials. A plurality ofcracks had formed with these spontaneously nucleated regions beingstarting points, and some of the cracks were able to be confirmed to bepenetrating from the bottom to the top surface of the oxidesuperconductors.

FIG. 3 is a schematic view of a structural photograph of the bottom ofan oxide superconductor of the comparative material. A plurality ofdark-colored, spontaneously nucleated region 13 with no crystalorientation are randomly formed in the body portion 12 of the oxidesuperconductor which has a general and homogeneous sandy pattern, and aplurality of cracks 14 are formed with the spontaneously reactingportions 13 being starting points.

In contrast, a spontaneously nucleated region was unable to be seen inthe bottom of the sample, and cracks did not form in the oxidesuperconductor sample of the present invention. FIG. 4 is a schematicview of a photograph of the bottom of the sample of the oxidesuperconductor of this example, and it was observed that the oxidesuperconductor 15 has a structure having a homogeneous and fine sandypattern.

Furthermore, a portion was confirmed in the bottom of the sample of theoxide superconductor of the present invention, at which a portion of theBaCuO₂ powder layer had melted and solidified after the layer meltedwith the partially molten precursor. It is presumed that cracks did notform since this melted and solidified portion had a surface structurehaving minute and fine pattern and demonstrated dense structuralcontinuity with the other portions.

In the next step, the aforementioned oxide superconductor of the presentinvention was cooled using liquid nitrogen in a 7 T (tesla) externalmagnetic field. After the magnetic field was removed, the trappedmagnetic field distribution of the surface was observed. The trappedmagnetic field distribution was confirmed to have a single peak free ofthe effects of cracks and so forth. In addition, the maximum value ofthe single peak of the trapped magnetic field was 1.1 T, thereby clearlydemonstrating that an extremely high magnetic field could be trapped.Next, when the trapped magnetic field on the bottom surface of the sameoxide superconductor was observed, the maximum value of the trappedmagnetic field was 1.25 T, thereby confirming that an oxidesuperconductor could be produced having a high trapped magnetic field onboth the top and bottom sides.

On the other hand, the trapped magnetic field distribution of the oxidesuperconductor of the comparative material decreased due to the presenceof cracking. This magnetic field distribution had multiple peaks, andthe maximum values of the peaks were low such that a maximum value onthe top surface of the oxide superconductor was 0.65 T and a maximumvalue on the bottom surface thereof was 0.2 T. This indicates that aplurality of cracks had formed in the bottom surface of the oxidesuperconductor of this comparative material, some of which hadpenetrated the oxide superconductor and reached the upper portionthereof.

Example 2

A pulverized sintered material having the compositional ratio ofSmBa₂Cu₃O_(7-x) (Sm123) and a pulverized oxide having a composition ofSm₂BaCuO₅ (Sm211) were weighed so that their blending ratio was 3:1.Moreover, 0.5% by weight of Pt and 10% by weight of silver oxide (Ag₂O)powder used for the purpose of inhibiting variations in mechanicalstrength of a bulk oxide superconductor to be produced were added, and amixture is obtained. This mixture was mixed and pulverized for 3 hoursin a powder mixing device to obtain a mixed powder of raw materials.

Uni-axial pressing was carried out on the aforementioned mixed powder ofraw materials at a pressure of 1.5 tons to mold into a pellet (diameter:40 mm, thickness: 13 mm). Subsequently, this pellet was subjected tocold isostatic pressing treatment at a pressure of 2 tons/cm² to obtaintwo precursors.

Next, a Y₂O₃ powder layer (intermediate layer) was formed on an alumina(Al₂O₃) boat (mount) in the same manner as Example 1, a BaCuO₂ powderlayer (substrate material) was formed thereon, and furthermore, theaforementioned precursor was placed thereon to obtain a sample of thepresent invention.

In addition, the same Y₂O₃ powder layer as that described above wasformed on an alumina (Al₂O₃) boat, and a mixed powder consisting of apowder having the compositional ratio of YbBa₂Cu₃O_(7-x) (Yb123) and apowder having a compositional ratio of Yb₂BaCuO₅ was spread thereonfollowed by placing the aforementioned precursor thereon to prepare acomparative material.

The sample and comparative material were placed in an argon gasatmosphere containing 1% oxygen, heated to 1090° C. to put theprecursors into a partially molten state, and then held at 1090° C. for1 hour. Subsequently, the sample and comparative material were cooled to1000° C. over 2 hours, and a Nd based seed crystal to which silver oxidehad not been added was placed on the top surface of the precursors inthe partially molten state followed by cooling to 990° C. over 5minutes. Next, the sample and comparative material were slowly cooled to975° C. at the rate of 1° C./hour, held for 70 hours at this temperature(975° C.), and then furnace-cooled to obtain disk-shaped oxidesuperconductors having a diameter of 30 mm and a thickness of 10 mm.

The resulting oxide superconductors were observed. A spontaneouslynucleated region (composite oxide portion) in which Yb preferentiallyreacted with the precursor in partially molten state was confirmed atmultiple locations on the bottom of the oxide superconductor of thecomparative material. A plurality of cracks had formed with thesespontaneously nucleated region being starting points.

In contrast, a spontaneously nucleated region was unable to be seen inthe bottom of the oxide superconductor sample of the present invention,and cracks did not form in the sample.

In the next step, the aforementioned oxide superconductor of the presentinvention was cooled using liquid nitrogen in a 7 T (tesla) externalmagnetic field. After the magnetic field was removed, the trappedmagnetic field distribution on the surface was observed. As shown inFIG. 5A, the trapped magnetic field distribution was able to beconfirmed to have a single peak free of the effects of cracks and soforth. In addition, the maximum value of the single peak of the trappedmagnetic field was 0.9 T, thereby clearly demonstrating that anextremely high magnetic field could be trapped.

Next, when the trapped magnetic field on the bottom of the same oxidesuperconductor was observed, as shown in FIG. 5B, the maximum value ofthe trapped magnetic field was 1.0 T, and it was able to be confirmedthat the trapped magnetic field distribution had a single peak and wasfree of cracks. Thus, an oxide superconductor was confirmed that thesuperconductor produced has a high trapped magnetic field on both thetop and bottom sides.

On the other hand, the results of the trapped magnetic fielddistribution measurements on the top and bottom of the oxidesuperconductor of the comparative material are shown in FIGS. 6A and 6B.The trapped magnetic field decreased due to the effects of cracking.These magnetic field distributions had multiple peaks, and the maximumvalues of the peaks were low such that a maximum value was 0.4 T on thetop surface of the oxide superconductor and a maximum value was 0.2 T onthe bottom surface of the oxide superconductor.

Example 3

A pulverized sintered materials having the compositional ratio ofGdBa₂Cu₃O_(7-x) (Gd123) and a pulverized oxide having a composition ofGd₂BaCuO₅ (Gd211) were weighed so that their blending ratio was 2:1.Moreover, 0.5% by weight of Pt, and 10% by weight of silver oxide (Ag₂O)powder used for the purpose of inhibiting variations in mechanicalstrength of the bulk-shaped oxide superconductor to be produced, wereadded to obtain a mixture. This mixture was mixed and pulverized for 3hours in a powder mixing device to obtain a mixed raw material powder.

Uni-axial pressing was carried out on the aforementioned mixed powder ofraw materials at a pressure of 1.5 tons to mold into apellet (diameter:40 mm, thickness: 13 mm). Subsequently, this pellet was subjected tocold isostatic pressing treatment at a pressure of 2 tons/cm² to obtaintwo precursors.

Next, a Y₂O₃ powder layer (intermediate layer) was formed on an alumina(Al₂O₃) boat in the same manner as Example 1, a BaCuO₂ powder layer(substrate material) was formed thereon, and furthermore, theaforementioned precursor was placed thereon to obtain a sample of thepresent invention.

In addition, the same Y₂O₃ powder layer as that described above wasformed on an alumina (Al₂O₃) boat, and a mixed powder consisting of apowder having the compositional ratio of YbBa₂Cu₃O_(7-x) (Yb123) and apowder having the compositional ratio of Yb₂BaCuO₅ was spread on thelayer followed by placing the aforementioned precursor thereon toprepare a comparative material.

These sample and comparative material were placed in an argon gasatmosphere containing 1% oxygen, heated to 1080° C. to put theprecursors into a partially molten state, and then held at 1080° C. for1 hour. Subsequently, the sample and comparative material were cooled to990° C. over 2 hours, and Nd based seed crystals to which silver oxidehad not been added were placed on the top surface of the precursors inthe partially molten state followed by cooling to 980° C. over 5minutes. Next, the sample and comparative material were slowly cooled to960° C. at the rate of 0.5° C./hour, held for 70 hours at thistemperature (960° C.), and then furnace-cooled to obtain disk-shapedoxide superconductors having a diameter of 30 mm and a thickness of 10mm.

The resulting oxide superconductors were observed. A spontaneouslynucleated region at which Yb preferentially reacted with the precursorin partially molten state was confirmed at multiple locations on thebottom of the oxide superconductor of the comparative material, and aplurality of cracks had formed with these portions at which reactionproceeded spontaneously as starting points.

In contrast, a spontaneously nucleated region was unable to be confirmedin the bottom of the sample, and cracks did not form in the oxidesuperconductor sample of the present invention.

When a trapped magnetic field was measured under the same conditions asExamples 1 and 2, the sample of the present invention was confirmed todemonstrate a trapped magnetic field distribution that had a single peakand was free of the effects of cracks and so forth. In addition, themaximum values of the single peak of the trapped magnetic field was 1.4T on the top surface side and 1.3 T on the bottom surface side, clearlydemonstrating that an extremely high magnetic field was able to betrapped. On the other hand, the trapped magnetic field of an oxidesuperconductor of the comparative material decreased due to the effectsof cracking, the trapped magnetic field distribution had multiple peaks,and the maximum values of the peaks were low such that a maximum valueon the top surface side of 0.7 T and a maximum value on the bottomsurface side of 0.4 T.

Here, the bulk-shaped oxide superconductor having the composition ofNdBa₂Cu₃O_(7-x) (Nd123) that is free of cracks and has a diameter of 30mm and thickness of 10 mm produced in the aforementioned Example 1 iscurrently the longest among Nd based oxide superconductor in the world.

In this type of RE-Ba—Cu—O based oxide superconductor, Nd based oxidesuperconductors are susceptible to cracking in the case of producing itusing a partial melting and solidification method, and what is more,cracks easily grow through the entire oxide superconductor once thecracks have formed. Here, even if, for example, a tiny crack forms inthe bottom side of a Y based oxide superconductor, the crack rarelypasses through the entire oxide superconductor. Because of thischaracteristic, large, bulk-shaped oxide superconductors of roughly 100mm diameter have actually been obtained using Y based oxidesuperconductors.

However, since Nd based oxide superconductors are susceptible to thespreading of cracks accompanying a partially melting and solidificationmethod in particular, it is has been extremely difficult in the priorart to obtain Nd based oxide superconductors which have a single peakand are free of cracks, and have a diameter of larger than 20 mm.

Under these circumstances, the present invention enables the productionof a large, bulk-shaped Nd based oxide superconductor having a diameterof 30 mm without the formation of cracks. Moreover, it has been clearlydemonstrated that one can obtain oxide superconductors that havesuperior characteristics in which they demonstrate only a single peak inthe trapped magnetic field distribution of samples thereof. The presentinvention has been shown to demonstrate extremely effective andremarkable effects in the technology for producing large, bulk-shapedoxide superconductors. Furthermore, the defect-free Nd based oxidesuperconductor having a diameter of 30 mm is currently the largest inthe world. The Nd based oxide superconductors which have a diameter of30 mm or more, and have a superior characteristic of demonstrating onlya single peak in a trapped magnetic field distribution, and providingthe same characteristics on the top and bottom sides of thesuperconductor, are currently unable to be achieved by using atechnology other than that of the present invention.

As has been described above, according to the present invention, aspontaneously nucleated region is not allowed to occur in a precursor bycarrying out a partially melting and solidification method on apartially molten precursor. The partially molten precursor is supportedwith a substrate material composed of a compound or pure metal that isable to melt in the partially molten precursor. Accordingly, an oxidesuperconductor can be obtained that is free of the formation of crackscaused by a difference of the thermal expansion coefficients.

INDUSTRIAL APPLICABILITY

In the present invention, when an oxide superconductor is produced bymelting and solidification of a precursor which is supported with asubstrate material that melts in the precursor but does not contain arare earth element, a molten and solidified portion that does notcontain a rare earth element but contains either or both of Ba and Cu isformed in a portion at which the precursor is supported. If an oxidesuperconductor having this type of molten solidified portion is producedby a partial melting and solidification method, since a portion at whichreaction proceeds spontaneously does not occur in the molten solidifiedportion, there are no portions having different coefficient of thermalexpansion in the resulting oxide superconductor, and a crack-free oxidesuperconductor is provided in which cracks caused by a difference in thecoefficients of thermal expansion do not form during solidification.

If an oxide superconductor is produced by supporting its bottom whilemelting and solidification, an oxide superconductor can be obtained thathas a molten solidified portion which is free of a portion at whichreaction proceeds spontaneously in its bottom.

1. A method for producing an oxide superconductor comprising: placing aprecursor of an oxide superconductor in a state where it is on asubstrate material containing pure metal or a compound which is meltablein the precursor when the precursor is in a partially molten state, andproducing the oxide superconductor by partial melting and solidifyingthe precursor in said state.
 2. The method for producing an oxidesuperconductor according to claim 1, wherein the substrate materialconsists of a pure metal or compound which is meltable in the partiallymolten precursor uniformly and which does not allow formation of aportion, at which a reaction proceeds spontaneously to form stressconcentration cracks resulting from a difference in the coefficients ofthermal expansion between the substrate material and the oxidesuperconductor.
 3. The production method of an oxide superconductoraccording to claim 1, wherein the oxide superconductor is an RE-Ba—Cu—Obased oxide superconductor, and the RE represents a rare earth element,and the substrate material is a material that contains Ba or Cu in amolten state but does not contain a rare earth element.
 4. Theproduction method of an oxide superconductor according to claim 1,wherein, after placing an intermediate layer containing at least oneselected from the group consisting of Y₂O₃, Yb₂O₃, Er₂O₃, Ho₂O₃, Dy₂O₃,Eu₂O₃, Sm₂O₃, Gd₂O₃, ZrO₂, Al₂O₃, BaZrO₃, MgO and yttrium stabilizedzirconia on a mount made of a heat-resistant material, the substratematerial is placed on the mount, and the precursor of the oxidesuperconductor is placed on the substrate material for conductingpartial melting and solidification of the precursor.
 5. The productionmethod of an oxide superconductor according to claim 1, wherein thesubstrate material contains at least one selected from the groupconsisting of a pure metal of Ba or Cu, oxides, composite oxides,carbonates, sulfides, sulfates, chlorides, hydroxides and nitrates of Baor Cu.
 6. The production method of an oxide superconductor according toclaim 5, wherein the oxide, composite oxide, carbonate, sulfide,sulfate, chloride, hydroxide or nitrate of Ba or Cu is BaO, CuO, Cu₂O,BaCuO₂, BaCO₃, CuCO₃, BaS, CuS, BaSO₄, CuSO₄, BaCl₂, CuCl, CuCl₂,Ba(OH)₂, Cu(OH)₂, Ba(NO₃)₂ or Cu(NO₃)₂.
 7. The production method of anoxide superconductor according to any one of claims 3 to 6, wherein thesubstrate material comprises at least one selected from noble metalsincluding Ag, Au, Pt and Pd and their oxides.
 8. A substrate materialfor supporting a precursor of an oxide superconductor, wherein thesubstrate material is used for a process for producing an oxidesuperconductor by solidification from a partially molten state of aprecursor of an RE-Ba—Cu—O based oxide superconductor, and RE representsa rare earth element, and the substrate material is a material whichcontains Ba or Cu but does not contain a rare earth element in thepartially molten state.
 9. The substrate material for supporting aprecursor of an oxide superconductor according to claim 8, wherein thesubstrate material is an aggregate of powder.
 10. The substrate materialfor supporting a precursor of an oxide superconductor according to claim8, wherein the substrate material contains at least one selected fromthe group consisting of a pure metal of Ba or Cu, an oxide, compositeoxide, carbonate, sulfide, sulfate, chloride, hydroxide and nitrate ofBa or Cu.
 11. The substrate material for supporting precursor of anoxide superconductor according to claim 10, wherein the oxide, compositeoxide, carbonate, sulfide, sulfate, chloride, hydroxide and nitrate ofBa or Cu is BaO, CuO, Cu₂O, BaCuO₂, BaCO₃, CuCO₃, BaS, CuS, BaSO₄,CuSO₄, BaCl₂, CUCl, CUCl₂, Ba(OH)₂, Cu(OH)₂, Ba(NO₃)₂ or Cu(NO₃)₂. 12.The substrate material for supporting a precursor of an oxidesuperconductor according to any one of claims 8 to 11, wherein thesubstrate material contains at least one selected from the groupconsisting of noble metals including Ag, Au, Pt and Pd, and theiroxides.
 13. An RE-Ba—Cu—O based oxide superconductor, wherein RErepresents a rare earth element, and the oxide superconductor includes aportion, which is solidified after melting and contains one of or bothof Ba and Cu but does not contain a rare earth element, on an outsideface of the oxide superconductor, and the solidified portion correspondsto a substrate material which is used for supporting a precursor of theRE-Ba—Cu—O based oxide superconductor when the precursor was melted andsolidified.
 14. The oxide superconductor according to claim 13, whereinat least one selected from noble metals including Ag, Au, Pt and Pd isadditionally contained in the solidified portion.
 15. The oxidesuperconductor according to claim 13, wherein the solidified portion isformed in the bottom of the oxide superconductor.
 16. The oxidesuperconductor according to any of claims 13 to 15, wherein trappedmagnetic field distributions on the top side and bottom side of theoxide superconductor are similar.